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Introduction to Ceylon Part 6

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This is the sixth installment in a series of articles introducing the Ceylon language. Note that some features of the language may change before the final release.

Defining generic types

We've seen plenty of parameterized types in this series of articles, but now let's explore a few more details.

Programming with generic types is one of the most difficult parts of Java. That's still true, to some extent, in Ceylon. But because the Ceylon language and SDK were designed for generics from the ground up, Ceylon is able to alleviate the most painful aspects of Java's bolted-on-later model.

Just like in Java, only types and methods may declare type parameters. Also just like in Java, type parameters are listed before ordinary parameters, enclosed in angle brackets. 

shared interface Iterator<out Element> { ... }
class Array<Element>(Element... elements) satisfies Sequence<Element> { ... }
shared Entries<Natural,Value> entries<Value>(Value... sequence) { ... }

As you can see, the convention in Ceylon is to use meaningful names for type parameters.

Unlike Java, we always do need to specify type arguments in a type declaration (there are no raw types in Ceylon). The following will not compile: 

Iterator it = ...;   //error: missing type argument to parameter Element of Iterator

We always have to specify a type argument in a type declaration: 

Iterator<String> it = ...;

On the other hand, we shouldn't need to explicitly specify type arguments in most method invocations or class instantiations. In principle it's very often possible to infer the type arguments from the ordinary arguments. The following code should be possible, just like it is in Java: 

Array<String> strings = Array("Hello", "World");
Entries<Natural,String> entries = entries(strings);

But we haven't yet figured out what exactly the type inference algorithm will be (probably something involving union types!) and so the Ceylon compiler currently requires that all type arguments be explicitly specified like this: 

Array<String> strings = Array<String>("Hello", "World");
Entries<Natural,String> entries = entries<Natural,String>(strings);

On the other hand, the following code does already compile: 

local strings = Array<String>("Hello", "World");
local entries = entries<Natural,String>(strings);

The root cause of very many problems when working with generic types in Java is type erasure. Generic type parameters and arguments are discarded by the compiler, and simply aren't available at runtime. So the following, perfectly sensible, code fragments just wouldn't compile in Java: 

if (is List<Person> list) { ... }
if (is Element obj) { ... }

(Where Element is a generic type parameter.)

A major goal of Ceylon's type system is support for reified generics. Like Java, the Ceylon compiler performs erasure, discarding type parameters from the schema of the generic type. But unlike Java, type arguments are supposed to be reified (available at runtime). Of course, generic type arguments won't be checked for typesafety by the underlying virtual machine at runtime, but type arguments are at least available at runtime to code that wants to make use of them explicitly. So the code fragments above are supposed to compile and function as expected. You will even be able to use reflection to discover the type arguments of an instance of a generic type.

The bad news is we haven't implemented this yet ;-)

Finally, Ceylon eliminates one of the bits of Java generics that's really hard to get your head around: wildcard types. Wildcard types were Java's solution to the problem of covariance in a generic type system. Let's first explore the idea of covariance, and then see how covariance in Ceylon works.

Covariance and contravariance

It all starts with the intuitive expectation that a collection of Geeks is a collection of Persons. That's a reasonable intuition, but especially in non-functional languages, where collections can be mutable, it turns out to be incorrect. Consider the following possible definition of Collection: 

shared interface Collection<Element> { 
    shared formal Iterator<Element> iterator(); 
    shared formal void add(Element x);
}

And let's suppose that Geek is a subtype of Person. Reasonable.

The intuitive expectation is that the following code should work: 

Collection<Geek> geeks = ... ; 
Collection<Person> people = geeks;    //compiler error 
for (Person person in people) { ... }

This code is, frankly, perfectly reasonable taken at face value. Yet in both Java and Ceylon, this code results in a compiler error at the second line, where the Collection<Geek> is assigned to a Collection<Person>. Why? Well, because if we let the assignment through, the following code would also compile: 

Collection<Geek> geeks = ... ; 
Collection<Person> people = geeks;    //compiler error 
people.add( Person("Fonzie") );

We can't let that code by — Fonzie isn't a Geek!

Using big words, we say that Collection is nonvariant in Element. Or, when we're not trying to impress people with opaque terminology, we say that Collection both produces — via the iterator() method — and consumes — via the add() method — the type Element.

Here's where Java goes off and dives down a rabbit hole, successfully using wildcards to squeeze a covariant or contravariant type out of a nonvariant type, but also succeeding in thoroughly confusing everybody. We're not going to follow Java down the hole.

Instead, we're going to refactor Collection into a pure producer interface and a pure consumer interface: 

shared interface Producer<out Output> { 
    shared formal Iterator<Output> iterator();
}
shared interface Consumer<in Input> { 
    shared formal void add(Input x);
}

Notice that we've annotated the type parameters of these interfaces.

  • The out annotation specifies that Producer is covariant in Output; that it produces instances of Output, but never consumes instances of Output.
  • The in annotation specifies that Consumer is contravariant in Input; that it consumes instances of Input, but never produces instances of Input.

The Ceylon compiler validates the schema of the type declaration to ensure that the variance annotations are satisfied. If you try to declare an add() method on Producer, a compilation error results. If you try to declare an iterate() method on Consumer, you get a similar compilation error.

Now, let's see what that buys us:

  • Since Producer is covariant in its type parameter Output, and since Geek is a subtype of Person, Ceylon lets you assign Producer<Geek> to Producer<Person>.
  • Furthermore, since Consumer is contravariant in its type parameter Input, and since Geek is a subtype of Person, Ceylon lets you assign Consumer<Person> to Consumer<Geek>.

We can define our Collection interface as a mixin of Producer with Consumer. 

shared interface Collection<Element> 
        satisfies Producer<Element> & Consumer<Element> {}

Notice that Collection remains nonvariant in Element. If we tried to add a variance annotation to Element in Collection, a compile time error would result.

Now, the following code finally compiles: 

Collection<Geek> geeks = ... ; 
Producer<Person> people = geeks; 
for (Person person in people) { ... }

Which matches our original intuition.

The following code also compiles: 

Collection<Person> people = ... ; 
Consumer<Geek> geekConsumer = people; 
geekConsumer.add( Geek("James") );

Which is also intuitively correct — James is most certainly a Person!

There's two additional things that follow from the definition of covariance and contravariance:

  • Producer<Void> is a supertype of Producer<T> for any type T, and
  • Consumer<Bottom> is a supertype of Consumer<T> for any type T.

These invariants can be very helpful if you need to abstract over all Producers or all Consumers. (Note, however, that if Producer declared upper bound type constraints on Output, then Producer<Void> would not be a legal type.)

You're unlikely to spend much time writing your own collection classes, since the Ceylon SDK has a powerful collections framework built in. But you'll still appreciate Ceylon's approach to covariance as a user of the built-in collection types. The collections framework defines two interfaces for each basic kind of collection. For example, there's an interface List<Element> which represents a read-only view of a list, and is covariant in Element, and OpenList<Element>, which represents a mutable list, and is nonvariant in Element.

Generic type constraints

Very commonly, when we write a parameterized type, we want to be able to invoke methods or evaluate attributes upon instances of the type parameter. For example, if we were writing a parameterized type Set<Element>, we would need to be able to compare instances of Element using == to see if a certain instance of Element is contained in the Set. Since == is only defined for expressions of type Equality, we need some way to assert that Element is a subtype of Equality. This is an example of a type constraint — in fact, it's an example of the most common kind of type constraint, an upper bound. 

shared class Set<out Element>(Element... elements) 
        given Element satisfies Equality {
    ...

    shared Boolean contains(Object obj) { 
        if (is Element obj) {
            return obj in bucket(obj.hash);
        }
        else {
            return false;
        }
    }

}

A type argument to Element must be a subtype of Equality. 

Set<String> set = Set("C", "Java", "Ceylon"); //ok
Set<String?> set = Set("C", "Java", "Ceylon", null); //compile error

In Ceylon, a generic type parameter is considered a proper type, so a type constraint looks a lot like a class or interface declaration. This is another way in which Ceylon is more regular than some other C-like languages.

An upper bound lets us call methods and attributes of the bound, but it doesn't let us instantiate new instances of Element. Once we implement reified generics, we'll be able to add a new kind of type constraint to Ceylon. An initialization parameter specification lets us actually instantiate the type parameter. 

shared class Factory<out Result>() 
        given Result(String s) {

    shared Result produce(String string) { 
        return Result(string);
    }

}

A type argument to Result of Factory must be a class with a single initialization parameter of type String. 

Factory<Hello> = Factory<PersonalizedHello>(); //ok
Factory<Hello> = Factory<DefaultHello>(); //compile error

A third kind of type constraint is an enumerated type bound, which constrains the type argument to be one of an enumerated list of types. It lets us write an exhaustive switch on the type parameter: 

Value sqrt<Value>(Value x) 
        given Value of Float | Decimal {
    switch (Value)
    case (satisfies Float) {
        return sqrtFloat(x);
    } 
    case (satisfies Decimal) {
        return sqrtDecimal(x);
    }
}

This is one of the workarounds we mentioned earlier for Ceylon's lack of overloading.

Finally, the fourth kind of type constraint, which is much less common, and which most people find much more confusing, is a lower bound. A lower bound is the opposite of an upper bound. It says that a type parameter is a supertype of some other type. There's only really one situation where this is useful. Consider adding a union() operation to our Set interface. We might try the following: 

shared class Set<out Element>(Element... elements) 
        given Element satisfies Equality {
    ...
    
    shared Set<Element> union(Set<Element> set) {   //compile error
        return ....
    }
    
}

This doesn't compile because we can't use the covariant type parameter T in the type declaration of a method parameter. The following declaration would compile: 

shared class Set<out Element>(Element... elements) 
        given Element satisfies Equality {
    ...
    
    shared Set<Object> union(Set<Object> set) { 
        return ....
    }
    
}

But, unfortunately, we get back a Set<Object> no matter what kind of set we pass in. A lower bound is the solution to our dilemma: 

shared class Set<out Element>(Element... elements) 
        given Element satisfies Equality {
    ...
    
    shared Set<UnionElement> union(Set<UnionElement> set) 
            given UnionElement abstracts Element {
        return ...
    }
    
}

With type inference, the compiler chooses an appropriate type argument to UnionElement for the given argument to union(): 

Set<String> strings = Set("abc", "xyz") ; 
Set<String> moreStrings = Set("foo", "bar", "baz"); 
Set<String> allTheStrings = strings.union(moreStrings);
Set<Decimal> decimals = Set(1.2.decimal, 3.67.decimal) ; 
Set<Float> floats = Set(0.33, 22.0, 6.4); 
Set<Number> allTheNumbers = decimals.union(floats);
Set<Hello> hellos = Set( DefaultHello(), PersonalizedHello(name) ); 
Set<Object> objects = Set("Gavin", 12, true); 
Set<Object> allTheObjects = hellos.union(objects);

There's more...

I was about to start talking about sequenced type parameters, the foundation of Ceylon's typesafe metamodel. But I realize I already hit my word limit. If you're really impatient, you can skip forward to Part 8.

In Part 7 we're going to back up a bit and cover a couple of topics that got kinda glossed over.

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